Properties, such as wear resistance, oxidation resistance, corrosion resistance, heat resistance, electrical or thermal conductivity, mechanical strength, and the like, which are inadequate in a single material can be supplemented by combining two or more materials which have different properties. Furthermore, it is possible to impart new functions such as magnetic properties, self-lubricating properties, optical properties, piezoelectric properties, thermoelectric properties, insulating properties, thermal conductivity, thermal insulation properties, dielectric properties, expansion properties, free cutting properties, electrical conductivity, and the like, i.e. properties which cannot be realized with each single substance alone. Consequently, composite materials in which various materials are combined have been investigated for realizing prescribed properties.
Especially with composite materials in which particles, whiskers or fibers, for example, which are comprised of a different material are dispersed in a base material (matrix), the dispersion material fulfills the role of realizing mechanical and functional properties and so it is possible to design a wide range of materials which have conjoint properties which cannot be obtained with single materials (monolithic materials), as mentioned above.
The conventional composite materials in which dispersion materials are dispersed are generally materials in which the dispersion material is dispersed uniformly in a matrix. The composite material can provide increased performance and functionality as a result of the addition of the dispersion material, but there is a problem in that the properties of the dispersion material cannot be exhibited satisfactorily because it is dispersed independently and randomly.
Dispersion of the dispersion material in the form of a continuous three-dimensional network has been proposed for resolving these problems. (Japanese Patent Kokai 60-243245, Japanese Patent Kokai 62-4750, Japanese Patent Kokai H1-119688, Japanese Patent Kokai H3-122066, Japanese Patent Kokai H3-174358, Japanese Patent Kokai H4-37667). It is described that these composite materials can exhibit satisfactorily the properties of the dispersion material since the dispersion material is distributed continuously.
The Japanese Patent Kokai 60-243245 discloses a "ceramic particle-reinforced metal composite material" comprising a porous ceramic skeleton formed by sintering a mixture of ceramic and whiskers comprising ceramic, and metal which has been impregnated into the gaps of said ceramic skeleton. It is described that the composite material has a dispersion material comprising a mixture of ceramic and ceramic whiskers dispersed in the form of a continuous skeletal structure in a metal matrix and so it is possible to make composite materials which have high fracture strength and high quality and which are strong against thermal shock.
The Japanese Patent Kokai 62-4750 discloses a "positive temperature coefficient composition and a method for its manufacture" wherein the composition comprises crystalline polymer and short carbon fibers of average length from 0.05 mm to 1 mm and of diameter from 3 .mu.m to 20 .mu.m. It is described that this composition has chains of a three-dimensional micro-network structure of short carbon fibers in a polymer matrix and so it is possible to reduce the amount of short carbon fibers which is used and to form polymer compositions which are cheap and which have excellent PTC characteristics.
The Japanese Patent Kokai H1-119688 discloses a "resin molded electrode and a method for its manufacture" wherein the resin molded electrode has particles of an electrically conductive metal such as lead dispersed continuously in a network form in a substrate comprising thermoplastic resin. It is described that this electrode has excellent corrosion resistance and mechanical strength, and it can be made cheaply.
The Japanese Patent Kokai H3-122066 discloses an "aluminum impregnated type silicon carbide composite material and a method for its manufacture" wherein the composite material has a skeleton which is formed of a porous body of low density silicon carbide and in which aluminum is retained in the pores of this skeleton. It is described that this composite material has aluminum impregnated into the continuous pores of a silicon carbide porous body which has continuous pores and so it is possible to form a composite material which is light, and which has excellent strength, heat resistance and wear resistance.
The Japanese Patent Kokai H3-174358 discloses a "composite material comprising a continuous phase of silicon carbide and carbon" which has a structure comprising from 90 to 30 mol % carbon and from 10 to 70 mol % silicon carbide and which together form a continuous phase. It is described that two components form a continuous phase in this composite material and so, even if the carbon component disappears as a result of oxidation, for example, it has a high bending strength and so it can retain its form.
The Japanese Patent Kokai H4-37667 discloses a "light weight high rigidity ceramic and its application" wherein the ceramic has a three-dimensional continuous network structure formed in a reactively sintered matrix. It is described that this light weight high rigidity ceramic is a composite ceramic structure which is of light weight and high rigidity, i.e. which has a high specific elastic modulus.
Problems to be Resolved by the Invention
However, the above-mentioned composite materials disclosed in Japanese Patent Kokai 60-243245, Japanese Patent Kokai 62-4750, Japanese Patent Kokai Hl-119688, Japanese Patent Kokai H3-122066, Japanese Patent Kokai H3-174358 and Japanese Patent Kokai H4-37667 are such that in all cases the strength of the composite is determined by the strength of either the base material or the dispersion material whichever is lower and the degree of density, and so it is difficult to achieve increased strength by simply dispersing the dispersion material in the form of a continuous three-dimensional network. Furthermore, internal stresses are generated continuously by the different thermal expansions between the base material and the dispersion material and so the mechanical and thermal shock resistances, for example, are reduced. Moreover, when preparing the composite material, a special process for the impregnation of the other material is required after forming the porous body which has a network structure of the dispersion material or the matrix. Consequently, long time is required for production, and this is not good for mass production and, moreover, there is a problem in that increasing the density is difficult.
Furthermore, for the light weight and high rigidity ceramics disclosed in Japanese Patent Kokai H4-37667, there is disclosed a method in which the dispersion material is made into a network using a powder comprised of a ceramic powder attached to a crushed metal powder or atomized powder, but since the dispersion material is dispersed continuously, the sinterability is poor and there is a problem in that, as a result of this, the strength is low.
Disclosure of the Invention
Object of the Invention
An object of this invention is to provide a composite material in which the properties of the dispersion material and the base material can be exhibited satisfactorily without any worsening of the mechanical properties, and a method for the manufacture.
Constitution of the Invention
A composite material of this invention comprises: a large number of composite material cells, as structural units of the composite material, each comprising a first phase composed of a base material and a second phase composed of a dispersion material surrounding said first phase discontinuously; and comprising a matrix comprising the base material and the dispersion material dispersed in the matrix, the dispersion material being dispersed discontinuously in the form of a three-dimensional network in the composite material; the dispersion materials of the composite material cells being combined to form a composite material skeletal part, thereby exhibiting properties of the dispersion material without reducing the strength of the matrix owing to the skeletal part, and improving strength characteristics of the composite material owing to the sleketal part serving as a resistance to external stress.
Function of the Invention
The mechanism by which a composite material of this invention realizes these excellent effects is as yet unclear, but it is thought to be as indicated below.
A composite material of this invention has, as structural units, composite material cells each omprising a first phase comprising the material of the base material and a second phase comprising the dispersion material which is formed in such a way as to surround said first phase discontinuously. By making the composite material cells into structural units, the aforementioned second phase forms a strong skeleton or path around the aforementioned first phase and the matrix is reinforced by this, and it is possible to inhibit softening of the material at high temperatures and dislocation or element diffusion movement. Furthermore, it is thought that in those cases where a material for increasing thermal or electrical conductivity is selected for the dispersion material, the thermal or electrical conductivity is also imparted by the formation of paths by the second phase.
Furthermore, because the composite materials of this invention have the aforementioned composite material cells as structural units and the aforementioned dispersion material is dispersed discontinuously in the form of a three-dimensional network in the composite material, which is to say because it is dispersed discontinuously in the form of a three-dimensional network in the composite material, it can be concluded that, as indicated below, a synergistic effect due to the two effects, namely the effect of reinforcement due to the dispersion material (dispersed phase), such as particles or whiskers or fibers for example, itself, and the reinforcing effect due to the skeletal structure of the dispersion material is obtained.
At room temperature, high stress handled by the strong base material rather than the dispersion material and, at the same time, the movement of dislocations is prevented by the dispersion material which has been dispersed in the form of a three-dimensional network in the composite material, and crack growth can be suppressed. Furthermore, at high temperatures, softening and deformation of the composite material is suppressed by the three-dimensional network-like dispersion structure skeleton of the dispersion material, and grain boundary sliding between crystal grains and the movement of dislocations is prevented by the dispersion material, and the fracture strength and creep properties can be improved. In particular, since the dispersion material is dispersed discontinuously, even if cracks are formed in the dispersion material or along the boundaries between the dispersion material and the base material, propagation is more difficult than in a case where the dispersion material is dispersed continuously in the form of a three-dimensional network. Furthermore, thermal or mechanical shocks are relieved. Moreover, because of the high degree of density, pores which become sources of failure are less liable to form in the composite material and so decreasing in strength due to pore formation is not liable to occur. Effective reinforcement at room temperature and at high temperature is possible in this way.
That is to say, when a dispersion material has been used for reinforcement purposes, said dispersion agent reinforces the composite material by means of the dispersion material such as particles or whiskers for example in the base material and also forms a skeletal structure in which it is dispersed discontinuously in the form of a three-dimensional network in the composite material, and pores, for example, which are the main cause of a lowering of strength are not formed as a result of the formation of said structure. Such a skeletal structure can suppress the crack extension and the movement of dislocations between adjoining network meshes by means of the dispersion material itself when a high stress is applied. By this means it is possible to increase strength and toughness. Furthermore, a skeletal structure which has a high heat resistance can be formed when a dispersion material which is highly heat resistant is used, and this skeleton can suppress softening and deformation of the base material. Moreover, the movement of dislocations and grain boundary sliding due to softening of the crystal grain boundaries can be suppressed by the dispersion material itself and so it is possible to improve the instantaneous fracture strength and creep resistance at high temperature. In particular, even if cracks extend in the dispersion material or along the boundary between the dispersion material and the base material, because the dispersion material is dispersed discontinuously, there is no path along which the crack can generally extend and grow as they would in a continuous phase, and propagation is difficult.
When a dispersion material which imparts functionality is used as described above, higher density of the composite material is attained as compared with the dispersion in a continuous network form, and pores which become fracture origins are less liable to be formed, and thus it is possible to impart functionality without reducing the strength.
Furthermore, since the dispersion material is arranged in the form of a network mesh, it is possible to exhibit the properties of the dispersion material more strongly than with a uniformly dispersed system. Moreover, the amount of the dispersion material added can be reduced when compared with that of a continuous network structure.
Moreover, a change in the spacing of the dispersion material produced by a difference in the thermal expansion between the base material and the dispersion material can also be used for measuring temperature.
As has been outlined above, the composite materials of this invention can be considered to be materials made as composite materials in which the properties of the dispersion material are exhibited satisfactorily without adversely affecting the mechanical properties of the base material.
Effect of the Invention
The composite materials of this invention enable the properties of the dispersion material to be exhibited satisfactorily without adversely affecting the mechanical properties of the base material.